1. Field of the Invention
The present invention relates to electronic components such as semiconductor device and substrate that can densely be mounted with a highly reliable connection and further relates to a method and a structure for mounting semiconductor devices.
2. Description of the Background Art
In recent years, with increasing requirements for reduction in size and weight of electronic equipment and devices represented by cellular phone and mobile data equipment, semiconductor devices have been reduced in size and increased in density. Mounting structures are accordingly proposed, with the purpose of achieving smaller size and higher density of semiconductor devices, such as bare-chip mounting structure having an LSI chip directly mounted on a circuit board as well as so-called chip size package (CSP) structure with the entire size reduced by forming a semiconductor device as close as possible in shape to an LSI chip. In order to accomplish such a high mounting-density, a structure is employed where connection materials are arranged on one side of a semiconductor device.
In the mounting structures as described above, there is a difference in thermal expansion coefficient between the bare chip or CSP and a circuit board on which the bare chip or CSP is mounted. In a mounting process, a thermal stress generated at the connecting portion therebetween causes a thermal strain. The thermal strain leads to metal fatigue of connection materials so that cracks are generated in the connection materials which are consequently broken, possibly resulting in a malfunction of electronic components. This problem is serious since it becomes difficult, with the reduction in size and weight as well as increase in integration and pin number of semiconductor devices, to provide a satisfactory structure for alleviating the thermal stress that is enough to prevent the connection materials from breaking.
FIG. 13 shows a conventional CSP mounting structure. Referring to FIG. 13, a semiconductor device 105 is connected to a substrate 106 via connection materials 113. When a thermal stress is repeatedly exerted on the mounting structure, a crack 130 is produced in connection material 113 at a site closer to an electrode of semiconductor device 105 as shown in an enlarged cross sectional view of FIG. 14. Referring to FIG. 14, crack 130 runs to cross connection material 113. In the structure having the substrate on which components are mounted with a good solder connection and an optimized electrode design, the crack appears in the electrode opening as shown in FIG. 14 resulting in rupture. In other wards, it is seen that the maximum stress concentration occurs in the electrode opening due to the repeated thermal stress after mounting, and consequently the crack is opened. It is noted that the electrode of the semiconductor device for which the connection material is provided is an external electrode and is herein referred to simply as “electrode.” In some cases, a component consisting of a connection material and an electrode may be herein referred to as external electrode.
In order to address the problem discussed above, it is proposed in Japanese Patent Laying-Open No. 10-173006 to use a composite ball as a connection material between a semiconductor device and a substrate, the composite ball containing a resin ball with a low modulus of elasticity. The resin ball within the composite ball can alleviate the thermal stress caused by the difference of thermal expansion coefficient between the semiconductor device and the substrate, and accordingly enhance the reliability of the connecting portion.
The composite ball is also proposed in Japanese Patent Laying-Open No. 7-45664 disclosing that balls of different types are arranged in respective connection materials of external electrodes. The balls of different types can thus be arranged to remarkably decrease the number of processes such as soldering process. It is further proposed in Japanese Patent Laying-Open No. 8-213400 that a ball of a low modulus of elasticity is provided in a connection material of an external electrode. According to this; a force exerted on a connecting portion in a bonding process is absorbed by the ball of the low elastic modulus in the connection material of the external electrode to prevent a crack from opening in the connection material of the external electrode.
These methods are proposed for enhancing the reliability of the connecting portion in the mounting structure while they have a problem as described below. FIG. 15 is a cross sectional view of a conventional connecting portion of a mounting structure. Referring to FIG. 15, an electrode 104 of a semiconductor chip 105 is electrically connected to a connection terminal 107 of a circuit board 106 via a conductor 102 covering a resin ball 101. Short circuit between electrode 104 on semiconductor chip 105 and an adjacent electrode or the like is prevented by a protection film 125. With a similar purpose, a protection film 126 of circuit board 106 is provided. When this mounting structure is applied for use, a repeated thermal stress causes a crack 130 running along the interface between resin ball 101 and conductor 102 that constitute a composite connection material as shown in FIG. 16. In the mounting structure merely including the resin ball of a low modulus of elasticity in the connection material, the crack as shown in FIG. 16 is generated. It is confirmed through experiments by the inventors of the present invention that the lifetime of this structure is shorter than any structure without resin ball. In the mounting structure as shown in FIG. 16, the stress concentrates on a surface where different materials meet, namely on and around the interface between the resin ball of the low elastic modulus and the metal (conductor). The resin and metal are joined in this region which is accordingly mechanically weak. If the metal on the surface of the resin ball has a multi-layer structure, diffusion between the metal layers produces a brittle inter-metallic compound on the interface. Due to such a mechanically weak region, a crack is opened at an early stage.
As shown in FIG. 17, if an electrode on a substrate and the external electrode mentioned above are mounted without a sufficient alignment with each other, the semiconductor device and substrate are connected with a positional deviation with respect to each other. Consequently, resin ball 101 of the low elastic modulus in the connection material connecting electrodes 104 and 107 cannot be covered uniformly with metal (conductor) 102 as seen in the region indicated by D in FIG. 17. Then, if a certain force is exerted on the connection material, the stress concentrates on the thinnest portion of the conductor covering the resin ball and thus a crack is generated at an early stage. Here, it is supposed that a resin ball is used for an electrode of a CSP of 0.8 mm pitch which is mounted on a substrate. In this case, a crack is produced when the center of an electrode of a semiconductor device and that of an electrode of the substrate are deviated from each other by at least 100 μm, since the resin ball cannot be covered uniformly with the conductor. Consequently, a failure occurs at an early stage.
Moreover, if all connection materials for electrodes of the semiconductor device are composite connection materials each formed of a resin ball and a conductor, the semiconductor device could not be mounted on the substrate with a high precision, which leads to an inconvenience as described below. Specifically, the electrodes of the semiconductor device and those of the substrate are connected by the connection materials, and the amount of conductors around resin balls is insufficient so that there is an insufficient self-alignment effect which moves the electrode center of the semiconductor device to allow it to correspond to the electrode center of the substrate. This is because the self-alignment effect is produced from the surface tension or the like of the conductors. Accordingly, the early-stage crack mentioned above is likely to occur.